Impact of glucose infusion on the structural and functional characteristics of adipose tissue and on hypothalamic gene expression for appetite regulatory neuropeptides in the sheep fetus during late gestation

Abstract

In the present study, our aim was to determine whether intrafetal glucose infusion increases fetal adiposity, synthesis and secretion of leptin and regulates gene expression of the 'appetite regulatory' neuropeptides neuropepetide Y (NPY), agouti-related peptide (AGRP), pro-opiomelanocortin (POMC) and cocaine- and amphetamine-regulated transcript (CART) and receptors (leptin receptor (OB-Rb) and melancortin 3 receptor (MC3R)) within the fetal hypothalamus. Glucose (50% dextrose in saline) or saline was infused (7.5 ml h(-1)) into fetal sheep between 130 and 140 days gestation (term = 150 +/- 3 days gestation). Glucose infusion increased circulating glucose and insulin concentrations, mean lipid locule size (532.8 +/- 3.3 microm2 versus 456.7 +/- 14.8 microm2) and total unilocular fat mass (11.7 +/- 0.6 g versus 8.9 +/- 0.6 g) of the perirenal fat depot. The expression of OB-Rb mRNA was higher in the ventromedial nucleus compared to the arcuate nucleus of the hypothalamus in both glucose and saline infused fetuses (F= 8.04; P < 0.01) and there was a positive correlation between expression of OB-Rb and MC3R mRNA in the arcuate nucleus (r= 0.81; P < 0.005). Glucose infusion increased mRNA expression for POMC, but not for the anorectic neuropeptide CART, or the orexigenic neuropeptides NPY and AGRP, in the arcuate nucleus of the fetal hypothalamus. These findings demonstrate that increased circulating glucose and insulin regulate gene expression of the neuropeptides within the fetal hypothalamus that are part of the neural network regulating energy balance in adult life.

Figure 1. Fetal plasma glucose (A), insulin (B) and change in plasma leptin concentrations compared to preinfusion values (C) in saline infused (open bars) and glucose infused (filled bars) groups between −96 and +216 h from the start of saline or glucose infusion

Asterisks denote a significant increase in plasma glucose and insulin concentrations compared to preinfusion concentrations.

Figure 2. Autoradiographs showing the distribution of expression of OB-Rb mRNA (A and B) and MC3R mRNA (C and D) in the hypothalamus of the sheep fetus at 140 ± 1 d gestation. OB-Rb and MC3R mRNA was expressed predominately in the arcuate nucleus (ARC) and ventromedial hypothalmaus (VMH) in both saline infused (left panel) and glucose infused (right panel) fetuses. The expression of OB-Rb mRNA was higher in the VMH compared to the ARC in both saline infused and glucose infused fetuses. E, different letters denote significant differences between mRNA expression of OB-Rb in the VMN and ARC. Scale bar, 1.3 mm

Figure 3. Autoradiographs showing the distribution of expression of POMC mRNA in the saline infused (A) and glucose infused (B) fetuses at 140 d gestation. POMC mRNA expression at 140 ± 1 d gestation was higher (P < 0.05) in glucose infused compared to saline infused fetuses (C). ARC, arcuate nucleus. Scale bar, 1.3 mm

Figure 4. Autoradiographs showing the distribution of expression of CART mRNA (A and B), NPY mRNA (D and E) and AGRP mRNA (G and H) in saline infused (left panel) and glucose infused (right panel) fetuses at 140 ± 1 d gestation. There was no difference in the localization or level of expression of CART, AGRP or NPY mRNA between the saline and glucose infused groups (C, F and I). ARC, arcuate nucleus; PVN, paraventricular nucleus; VMH, ventromedial hypothalamus; ME, median eminence; DMH, dorsomedial hypothalamus. Scale bar, 1.3 mm